How do I unwrap a model proportionally?
Average Island Scale

For this task I usually take advantage of the Average Island Scale operator available in the UVs menu. I don't know the math behind it, but the effect is to re-proportionate the uv island to the region extension of the real mesh.

Here's a basic example:



And here's an example with Suzanne UVs. I duplicated the same object, so the UV island are at the beginning overlapping, then I ran the command.